So far I have proof (facebook messages, emails) that he is cheating on her, whether he has slept with this woman or not I am unsure. Should I tell my mum or leave it? My mum would be distraught as they have been married for over 20 years plus she thinks he is being kind and caring (for once in his life). I don't know what to do.

My dad told me not to tell my mum he was going to leave her. I didn't tell her. Then he told her I knew and hadn't said anything. It's been 3 years, she hasn't forgiven me. Take from that what you want.